The conventional linear motion damping device, such as an oil pressure cylinder or air pressure cylinder, is generally formed of a cylindrical body which is provided therein with a dragging rod intended to be pushed or pulled. The dragging rod is provided with a resistance by a hydraulic oil, gas, or spring at the time when the dragging rod is engaged in the axial displacement. The linear motion damping device is widely used in various fields, such as an exercise machine, a hatchback door, a furniture, a structure door, a smoke exhausting window, etc. As the dragging rod of the linear motion damping device is displaced, the fluid contained in the cylindrical body is exerted on by a constant pressure, thereby resulting in a gradual sealing deterioration. The damping device is thus useless. The implication is that the load of the conventional linear motion damping device is often limited such that the speed at which the dragging rod displaces axially is compromised. For this reason, the sealing structure of a damping device with a large load must be specially designed at the expense of the structural simplicity of the damping device.